The present invention relates to a method of coupling a spliceable optical fibre to an optical component; a spliceable optical fibre; a preform for producing a spliceable optical fibre; a method of producing a spliceable optical fibre comprising drawing of the preform; a heat-treated spliceable optical fibre; an article comprising a spliceable optical fibre.
1. The Technical Field
In recent years a new class of optical fibres has appeared. The optical guiding mechanism in these fibres is provided by introducing a number of holes or voids in the optical fibres. These holes typically run parallel with the fibre and extend along the fibre length. Such fibres are generally described by A. Bjarklev et al. in “Photonic Crystal Fibres”, Kluwer Academic Publishers, 2003 (ISBN 1-4020-7610-X), which is referred to in the following as [Bjarklev et al.]).
The light guiding principle can either be based on Total Internal Reflection (TIR) similar to the guiding principle of traditional optical fibres (non-microstructured optical fibres, also termed ‘standard optical fibres’ in the following), which do not comprise such holes, or it can be based on the Photonic Band Gap (PBG) principle.
For TIR-based optical fibres, the core typically consists of solid glass, which has a larger refractive index than the effective refractive index of the surrounding cladding region, which includes a number of closely spaced holes.
For PBG-based optical fibres, the core is not limited to a solid material. It can be a hole, or a combination of a solid background material and holes, surrounded by a cladding region comprising a solid background material and holes arranged in a predetermined pattern therein. The refractive index of the core can take any value, since light guiding is given by the fact that light cannot propagate through a cladding region comprising a cladding material with patterned holes. Consequently, light is confined within the core. The cladding region typically comprises a cladding material and carefully arranged air holes of predetermined hole size, distance and pattern. However, generally the holes can be any so-called feature comprising a material having a refractive index different from that of the background material.
Both types of optical fibres rely on air holes, or features in the cladding, to give them their optical properties. In general, these types of optical fibres will in the following be called photonic crystal fibres (PCFs). Optical fibres of this type are also known as microstructured fibres, holey fibres, photonic band gap fibres, hole-assisted optical fibres, as well as other names may be used.
Recent PCFs have characteristics quite different from conventional, solid glass optical fibres and thus find applications in a range of different fields. To increase possible applications of these PCFs, the coupling technology applied is very important, both for coupling light between different optical fibres and for coupling light between PCFs and optical components.
2. Prior Art Disclosure
Transition from small core PCFs to standard optical fibers is generally difficult. Splice losses are typically high (≧0.3 dB—see e.g. Hansen et al., “Highly Nonlinear Photonic Crystal Fiber with Zero-Dispersion at 1.55 μm” Optical Fiber Communication Conference 2002 post deadline paper, 2002), and the mechanical strength is poor when short term heating (sometimes referred to as “cold” splices) is used.
Tapering of PCF may be used to provide low loss transition coupling from PCF to standard optical fibres (see e.g. WO00049435 or EP01199582). However, tapering is time-consuming and laborious work involving manufacturing of tapered optical fibre regions. Furthermore, due to significantly reduced fibre diameter (typically a few tens of micrometers), the strength of optical fibres with tapered regions is lower than for un-tapered optical fibres.
US 2002/0114574-A1 discloses a heating and stretching technique for partially or fully collapsing a microstructured optical fibre in a tapered form, or in a non-tapered form keeping the overall diameter about the same, and providing a resultant optical fibre exhibiting mode contraction or mode expansion, respectively. A microstructured fibre with a single cladding region (apart from an over-cladding) with a single background material is disclosed.